Stem cells are a class of undifferentiated cells which are able to produce undifferentiated or specialized cell types after cell division. There are two main sources:
- cells from the inner cell mass (ICM) formed during the blastocyst phase of embryological development (embryonic stem cells); and
- adult tissue (adult stem cells), also called somatic cells (non-germ cells, sperm or egg).
Embryonic and adult stem cells share some common characteristics. They both have the potential to develop into different cell types in the body for growth and development. They are the repair system after tissue damage and injuries. Three important characteristics of stem cells can be identified.
Three important characteristics of stem cells
- Stem cells have the ability to divide and renew through cell division for long periods of time, or even after a long period of dormancy (inactivity). The other cells, muscle cells or nerve cells, cannot replicate or proliferate. Stem cells can proliferate by cell division. Moreover, the stem cells are capable of long term self renewal, which means the stem cells remain unspecialized (the same as their parent stem cells) after many times of cell division.
- Stem cells are unspecialized (Figure 1.1). A stem cell does not have any tissue-specific structure to perform specialized functions, for example carrying oxygen molecules through the blood stream or transmitting nerve impulses. However, the stem cell can produce specialized cells such as nerve cells and blood cells.
- Stem cells can give rise to specialized cells after division, which is called differentiation (Figure 1.1). This process takes place in several stages and the cells become more specialized at each stage. The intermediate cell types are called committed progenitors, which will be discussed in a later section. There are certain signals both inside and outside the cell that trigger each step of the differentiation process. Internal signals are controlled by genes in the nucleus, and external signals include chemicals secreted by other cells, physical contact with the neighboring cells and molecules in the microenvironment (cytoplasm or tissue fluid). Apart from those signals triggering the differentiation process, investigation has revealed the involvement of epigenetics of DNA expression in cell and stem cell division. This differentiation process of stem cells helps to replenish those cells that cannot undergo any cell division, like blood cells and nerve cells.
Stem cells are unspecialized. They can self-renew by copying themselves to form identical unspecialized stem cells. Stem cells can also give rise to specialized cells after division. This process is called differentiation.
Now try the following self-test and check how much you have absorbed from what we have covered so far.
- Please give two characteristics of self renewing cells.
- What are the factors in living organisms that normally regulate stem cell proliferation and self renewal?
- What are stem cells? Please give two functions of stem cells.
Self-test 1 feedback
- Characteristics of self-renewing cells: (a) the ability to divide and renew through cell division for long periods of time, and (b) to remain unspecialized after many times of cell division
- Internal signals: genes in the nucleus and epigenetics, and External signals: chemicals secreted by other cells, physical contact with the neighboring cells and molecules in the microenvironment (cytoplasm or tissue fluid).
- Stem cells are unspecialized cells, which can divide continuously and maintain the undifferentiated state. The functions of stem cells are: they differentiate into different types of cells to (a) develop, and (b) repair the damaged cells.